Excessive activation of the N-Methyl-D-Aspartate receptor (NMDAR) and the neurotransmitter dopamine (DA) mediate neurotoxicity and neurodegeneration under many neurological conditions, including stroke, ischemia, and Huntington's diseases (HD). Among all forebrain DA targets, the medium spiny projection neurons (MSNs) in the striatum is the densest innervated cell population by DA. At the ultrastructural level, DA and glutamate axon terminals converge on the same dendritic spines of postsynaptic MSNs, forming "synaptic triads". Emerging evidence reveals a positive feedback loop between NMDAR and the DA D1 receptor, which co-localize and mutually potentiate one another. Mechanisms need to be in place to not only assure the proper localization of both receptor types, but also to balance the interaction between them. Synaptic scaffolds of the membrane-associated guanylate kinase (MAGUK) family have been proposed to stabilize NMDARs in the synapse, which when dysregulated, may have an adverse effect on neuronal survival. Our ongoing work now reveals that the prototypic MAGUK PSD-95, in addition to its classical role in NMDAR scaffolding, may also regulate D1 function and D1-NMDAR interaction through the formation of a NMDAR/PSD-95/D1 complex. When PSD-95 is deleted in a unique transgenic mouse strain in my laboratory, the mutant mice develop severe neurological impairments. The neurological phenotypes are robust, progressive, and dominant, resembling those displayed by mouse models of neurodegeneration. These mice provide a unique in vivo system to explore the potential role of PSD-95 in MSN susceptibility. Our overall hypothesis is that PSD-95 plays an important role in the synaptic organization of NMDARs and their interaction with the DA system to regulate the striatal NMDAR function, and impairments to the NMDARs in KO mice confer vulnerability to striatal MSNs. We will explore this hypothesis with the following Specific Aims. In Aim I, we hypothesize that cell death accompanies the neurological phenotypes observed in these mice and will investigate striatal neurodegeneration in these mice using a set of immunohistochemical and stereological techniques. In Aim II, we hypothesize that absence of the PSD-95 scaffold in the mutant mice impairs their striatal NMDAR system, and will determine the expression, subcellular localization, and functional properties of striatal NMDARs in presymptomatic young PSD-95 deficient mice using biochemical, ultrastructural, and brain slice electrophysiological approaches. Our preliminary data suggest differential dysregulations of different NMDAR subunit activity in the corticostriatal synapses of PSD-95 deficient mice. In Aim III, we hypothesize that the NMDAR impairment in the mutant mice renders these mice more sensitive to NMDAR activation and will investigate striatal susceptibility to NMDAR excitotoxicity in presymptomatic PSD- 95 deficient mice using an in vivo excitotoxicity model. This work promises to provide critical insights into the molecular mechanisms underlying striatal vulnerability in Huntington's Disease, and has broader implications for understanding the neurotoxicity involved in stroke and ischemia. PUBLIC HEALTH RELEVANCE: A transgenic mouse strain in my laboratory lacking PSD-95, a prototypical PDZ domain- containing, NMDA receptor and dopamine receptor-interacting protein, develops robust, progressive, and dominant neurological phenotypes resembling those displayed by mouse models of neurodegeneration. The purpose of this application is to explore this novel in vivo role of PSD-95 in NMDA receptor function, striatal neuron survival, and its susceptibility to NMDA receptor excitotoxicity. This work promises to provide critical insights into the molecular mechanisms underlying striatal neurodegeneration in Huntington's Disease, and has broader implications for understanding the neurotoxicity involved in stroke and ischemia.